Learn Turkish: Toplu yerlerde yüksek sesle konuşulmaz. - One does not speak with a loud voice in public places.

This is an impersonal passive, which is a very common structure in Turkish. You can add the passive suffix to an intransitive verb (a verb that doesn't take a direct object) to mean "one does" or "people in general do" something. So konuşulmaz literally means "it is not spoken," but it translates as "one does not speak" or "people shouldn't speak."

Why use the negative aorist tense (-maz) here instead of the present continuous (-mıyor)?

The negative aorist (-maz) is used because this sentence states a general rule, a habit, or a social norm. If you said konuşulmuyor, it would mean "no one is speaking loudly right now." Using konuşulmaz means "speaking loudly is not something that is generally done here."

I've learned that "halk" means public. Why does this sentence use "toplu" for public places?

While halk means "the public" (as in the people), toplu means "collective" or "gathered together" (related to the verb toplamak, to gather). Toplu yerler refers to communal spaces where people are gathered together. You will also see this word in toplu taşıma, which means "public transportation."

How exactly does "yüksek sesle" translate to "with a loud voice"?